CAPTCHA™, which stands for Completely Automated Public Turing tests to tell Computers and Humans Apart, are tests designed such that most humans can pass but not current computer programs.
Automated computer programs, known as “robots” or “bots”, have been used to process data with minimum human intervention. While bots can be useful in certain situations, bots may lead to abuses and/or security threats in other situations.
For example, students at Carnegie Mellon University and Massachusetts Institute of Technology (MIT) have wrote bots to automatically generate votes for an online poll asking which was the best graduate school in computer science, turning the online poll into a contest between voting bots.
For example, free email services have suffered bot attacks which sign up thousands of email accounts every minute. The accounts were then used as a launch pad for sending millions of unsolicited email messages.
The School of Computer Science at Carnegie Mellon University has the CAPTCHA™ project funded by the National Science Foundation (NSF) Aladdin Center. CAPTCHA™ tests may be used to prevent search engine bots from indexing certain web pages and prevent others from finding them easily. CAPTCHA™ tests may be used to prevent dictionary attacks in password systems. A few companies are marketing the idea of using CAPTCHA™ tests to fight email worms and spam through enforcing “I will only accept an email if I know there is a human behind the other computer.”
Scientist Alan Turing pioneered the concept of Turing test such that computers can be considered intelligent when they can fool a human into thinking they were having a typed conversation with another person rather than a machine.
A CAPTCHA™ test is a Turing test but designed to automatically tell human and machine apart based on the difference in capabilities between humans and the current state of art of computer programs. CAPTCHA™ tests are generated automatically and designed to be Public Turing tests which rely on randomness in the generation of the tests but neither on secret database nor on secret algorithm.
A typical CAPTCHA™ test presents a word rendered as an image. The word is distorted in shape and obfuscated with different colors and other features such as dots and lines. Most humans can pass the test, but blind and partially sighted people became the inadvertent victims in a war against bots of spammers when such CAPTCHA™ tests are used.